1) Field of the Invention
This invention relates to an optical waveguide device suitable for use with an optical communication system.
2) Description of the Related Art
With the increase of the data transfer rate in recent years, in the field of optical communication systems, development of an optical waveguide device for modulating a data signal from an electric signal into an optical signal is proceeding energetically.
FIG. 3 is schematic view showing an optical modulator 110 to which a conventional optical waveguide device 100 for performing such optical modulation as just described is applied. The optical modulator 110 shown in FIG. 3 is formed by connecting RF signal generators 106a and 106b and biasing power supplies 107a and 107b, which are hereinafter described, to the optical waveguide device 100.
In the optical waveguide device 100 shown in FIG. 3, a Mach-Zehnder type optical waveguide 102 including a light incoming side Y-branch waveguide 102a, linear waveguides 102b and 102c and a light outgoing side Y-branch waveguide 102d is formed on a substrate 101Z wherein LiNbO3 (lithium niobate) is cut in vertical to the Z-axis of crystal orientation or a Z-cut direction.
Further, reference characters 103a and 103b denote signal electrodes mounted on the linear waveguides 102b and 102c, respectively. The signal electrodes 103a and 103b are connected to separate RF (Radio Frequency) signal generators 106a and 106b and apply RF signals, whose phases are opposite each other, from the RF signal generators 106a and 106b as modulation signals, respectively.
Further, reference characters 104a and 104b denote bias electrodes provided separately from the signal electrodes 103a and 103b, respectively. The bias electrode 104a is connected to the power supply 107a which produces a DC voltage (+V0), and provides a voltage as a DC (Direct Current) bias component to a microwave applied to the signal electrode 103a. The bias electrode 104b is connected to the power supply 107b which produces another DC voltage (−V0) and provides a voltage as a DC bias component to a microwave applied to the signal electrode 103b. 
Further, reference numeral 105 denotes a ground electrode. The ground electrode 105 provides a ground potential as a reference for the voltages to be supplied from the signal electrodes 103a and 103b and bias electrodes 104a and 104b and is formed in a predetermined spaced relationship from the formation areas of the electrodes 103a, 103b, 104a and 104b. 
Consequently, the voltages (−V0 and +V0) whose absolute values are equal to each other but whose signs are opposite to each other with reference to the potential of the ground electrode 105 are applied to the bias electrodes 104a and 104b to control a drive point corresponding to the signal electrodes 103a and 103b, respectively.
In the case of such a dual electrode structure as shown in FIG. 3 wherein the signal electrodes 103a and 103b are formed on the linear waveguides 102b and 102c, respectively, since wavelength chirp in the linear waveguides 102b and 102c can cancel each other in the outgoing side Y-branch waveguide 102d, the dual electrode structure of FIG. 3 is useful to an optical modulator for long distance transmission in that distortion of a pulse waveform as modulation data can be suppressed.
FIG. 4 is a schematic view showing a second optical modulator 210 to which another conventional optical waveguide device 200 for performing light modulation is applied. In the optical waveguide device 200 shown in FIG. 4, a Mach-Zehnder type optical waveguide 202 having a branch width smaller than that in the optical waveguide device 100 described hereinabove with reference to FIG. 3 is formed on a substrate 101X wherein LiNbO3 (lithium niobate) is cut in vertical to the X-axis of crystal orientation or a X-cut direction. It is to be noted that the optical waveguide 202 includes a light incoming side Y-branch waveguide 202a, linear waveguides 202b and 202c, and a light outgoing side Y-branch waveguide 202d. 
In the optical waveguide device 200 shown in FIG. 4, since the electric field direction when a voltage is applied is different from that in the optical waveguide device 100 shown in FIG. 3, a single signal electrode 203 and a single bias electrode 204 are formed on a formation face for the linear waveguides 202b and 202c in parallel along and between the linear waveguides 202b and 202c. Consequently, similarly as in the case of FIG. 3, the signal electrode 203 can cancel wavelength chirp in the linear waveguides 202b and 202c. 
It is to be noted that reference numeral 205 denotes a ground electrode, 206 an RF signal generator connected to the signal electrode 203, and 207 a power supply connected to the bias electrode 204.
It is to be noted that, where such a substrate 101X which extends in parallel to the X-axis of crystal orientation as shown in FIG. 4 is used, it is difficult to establish velocity matching between the microwave and the light. Therefore, it is usually the case that, when high-rate data is to be modulated, an optical waveguide device having the configuration shown in FIG. 3 is utilized to form an optical modulator.
However, where an optical waveguide device having such a configuration as described hereinabove with reference to FIG. 3 is utilized to form an optical modulator, since it is obliged to secure some lengths for the signal electrodes 103a and 103b on the linear waveguide 102b and 102c, it cannot be avoided to form the bias electrodes 104a and 104b comparatively short.
Since the product of the operating point voltage and the length of the electrode is specified to a fixed value from the characteristic of the device, it cannot be avoided to set the driving voltage for each of the bias electrodes 104a and 104b having a small length, that is, the driving voltages of the power supplies 107a and 107b, to a comparatively high voltage. Further, where the dual electrode structure is used, there is a subject that it cannot be avoided to prepare a plurality of power supplies for supplying such comparatively high driving voltages as described above.